This invention relates to the use of borate-polyol complexes in ophthalmic compositions. In particular, these complexes are useful in aqueous ophthalmic compositions containing polyvinyl alcohol to prevent the formation of borate-polyvinyl alcohol complexes, which are water-insoluble.
Ophthalmic compositions are generally formulated to have a pH between about 4.5 and 8.0 (physiological pH). To achieve a pH in this range, a buffer is often included. Borate buffer is the buffer of choice for use in ophthalmic compositions, since it has some inherent antimicrobial activity and often enhances the activity of antimicrobials; however, when polyvinyl alcohol (PVA) is also an ingredient in the composition, borate and PVA form a water-insoluble complex which precipitates out of solution and acts as an irritant in the eye. The incompatibility of borate and PVA in contact lens solutions has been discussed in an article by P. L. Rakow in Contact Lens Forum, (June 1988), pages 41-46.
Since borate is incompatible with PVA, ophthalmic compositions containing PVA are generally buffered with acetate, phosphate or other buffers. There are disadvantages to using these alternative buffers: for example, acetate is a weak buffer (pK.sub.a of about 4.5), so a relatively large amount is needed, whereas phosphate is a good buffer, but it reduces the antimicrobial activity of preservatives. A need therefore exists for ophthalmic compositions which have a physiological pH, but whose antimicrobial efficacy is not compromised.